The disclosure of Japanese Patent Application No. 2000-325182 filed on Oct. 25, 2000, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates generally to a hybrid vehicle that is driven by a suitable combination of an internal combustion engine and an electric motor, in view of savings of energy resources and environmental protection, and in particular to such a hybrid vehicle, and method of operating same, in which the quantity of NOx discharged or released into the atmosphere is reduced.
2. Description of Related Art
It has been proposed to temporarily stop an internal combustion engine of a vehicle, such as an automobile, during an operation of the vehicle, for the sake of savings of energy resources and environmental protection, when certain conditions under which the engine is allowed or desired to be temporarily stopped are established. In fact, this technology has been implemented in some types of automobiles. The conditions for temporarily stopping the engine may be established when, for example, the vehicle stops at a red traffic light, or the vehicle stops or travels at a considerably low speed because of a traffic jam, or the like.
The above-described technology of temporarily stopping the engine may be employed in a hybrid vehicle including an engine, an electric motor, a generator and a battery. In the hybrid vehicle, power can be transmitted among the engine, the motor and the generator, and the motor uses the battery as an electric power supply. The hybrid vehicle may be driven by one or both of the engine and the motor. Namely, the hybrid vehicle runs by means of a selected one or both of the output of the engine and the output of the motor. Furthermore, the generator is driven when appropriate by utilizing one or both of the output of the engine and the inertia of the running vehicle, so as to charge the battery. A known example of a hybrid vehicle is disclosed in Japanese Laid-open Patent Publication No. 7-255104.
The hybrid vehicle as described above has been developed so as to achieve improved environmental protection as one of its important objects. To accomplish this object, an exhaust system of the engine is provided with a catalytic converter including a catalyst, such as a three-way catalyst, for removing harmful substances, such as NOx, CO and HC, which are contained in exhaust gases emitted from the engine.
The hybrid vehicle as described above is able to operate appropriately even with a considerably high frequency of stopping of the engine, provided that the combination of the engine and the electric motor, along with the generator and the battery, operates in a well-balanced manner in accordance with operating or driving circumstances of the vehicle. However, the following problem arises if the catalytic converter is provided in the exhaust system of the hybrid vehicle that undergoes frequent stoppage of the engine. Namely, while the engine is stopped by initially cutting off fuel supply into the engine, the engine that has been rotating does not immediately stop rotating due only to the cut-off of the fuel supply, but rather keeps rotating for a while due to inertia. During the continued rotation of the engine, intake air that is rich in oxygen is discharged as it is into the catalytic converter, and the oxygen is trapped or stored in the catalytic converter each time the engine is stopped. If the engine is re-started in this condition, HC or CO that is fed to the catalytic converter along with NOx in the exhaust gas tends to react with the oxygen stored in the catalytic converter, rather than reacting with NOx under the effect of the catalytic converter so as to convert NOx into harmless N2. Consequently, an undesirably large quantity of NOx is discharged from the engine without being removed by the catalytic converter.
It is therefore an object of the invention to provide a hybrid vehicle that is able to reduce the quantity of NOx emitted from an engine, by preventing a catalytic converter from storing a large quantity of oxygen, and to a method of operating same.
To accomplish the above and/or other objects, there is provided according to one aspect of the invention a hybrid vehicle including: (a) a power train including an internal combustion engine, an electric motor, and a generator, which are operatively connected to each other, at least one of an output of the engine and an output of the electric motor being utilized for running the hybrid vehicle, (b) a battery electrically connected to the electric motor and to the generator, the electric motor using the battery as an electric power supply, at least one of the output of the engine and inertia of the running hybrid vehicle being utilized for driving the generator so as to charge the battery, and (c) a catalytic converter disposed in an exhaust system of the engine so as to purify an exhaust gas emitted from the engine. In the hybrid vehicle, a controller determines whether a condition for stopping the engine is established, based on an operating state of the vehicle, and cuts off fuel supply to the engine when the condition for stopping the engine is established. After cutting off fuel supply to the engine, a load (e.g., a load of the generator) is applied to the engine so as to restrict rotation of the engine. Thus, the generator gives resistance to rotation of the engine, thereby preventing the engine from rotating due to inertia soon after the fuel supply is cut off. Since a large quantity of intake air that is rich in oxygen is thus prevented from being discharged from the rotating engine into the catalytic converter, it is possible to reduce the quantity of NOx that is released to the atmosphere without being removed by the catalytic converter upon a re-start of the engine.
In one preferred embodiment of the invention, the temperature of the catalytic converter is determined through detection or estimation, and the engine is kept rotating (i.e., the fuel supply to the engine is not cut off) irrespective of establishment of the condition for stopping the engine if the temperature of the catalytic converter is higher than a predetermined threshold level. When the vehicle is in an operating state in which the catalyst temperature is higher than the predetermined threshold level, the catalytic converter is more likely to trap oxygen contained in the exhaust gas. If the condition for stopping the engine is established while the vehicle is in this operating state, a control operation to stop the engine is not performed or is temporarily suspended. Thus, oxygen contained in the intake air is prevented from being trapped by and stored in the catalytic converter, and the quantity of NOx that is released to the atmosphere without being removed by the catalytic converter is reduced. In this case, the total time during which the engine is stopped may be reduced, but it is still advantageous to keep the engine rotating rather than stopping the engine, in view of the savings of the energy resources and the environmental protection.
In another preferred embodiment of the invention, the hybrid vehicle further includes a shut-off valve disposed upstream of the catalytic converter in the exhaust system of the engine, and the shut-off valve is closed when the engine is stopped. With the shut-off valve thus closed, exhaust gas that is emitted during idling of the engine after cut-off of fuel supply is blocked at a point upstream of the catalytic converter, and thus oxygen contained in the exhaust gas is prevented from being trapped by and stored in the catalyst. When the shut-off valve is opened upon a re-start of the engine, the same quantity of oxygen as that blocked by the shut-off valve flows into the catalytic converter, but the ability of the catalyst to remove NOx from the exhaust gas is not deteriorated so much as that in the case where no shut-off valve is provided upstream of the catalytic converter and excessive oxygen is stored in the catalytic converter before the engine is re-started.